1. Field of the Invention
This invention relates to a molding compound useful in integrated circuit packaging. More particularly, this invention relates to a molding compound useful in an integrated circuit package having increased thermal conductivity whereby heat generated by the encapsulated die, or chip, may be more efficiently transmitted to the surface of the package for dissipation.
2. Description of the Prior Art
Conventionally integrated circuit dice are encapsulated in a filled plastic, such as a filled epoxy resin. Heat generated by the encapsulated die is dissipated by the use of copper lead frames to conduct the heat to the outside of the package.
However, with increasing densification of circuitry in very large scale integration (VLSI) technology, more than 10.sup.5 devices may be placed on a single chip. The amount of heat which may be generated, particularly if a sizable amount of these devices are high power requirement bipolar-type devices, necessitates improvements in the heat dissipation capabilities of the integrated circuit package.
Since the die has almost equal contact with the lead frame and the encapsulation material, and heat flowing to the copper lead frame is transferred to the encapsulation material, and the encapsulant aids heat flow from the die paddle to the fingers, it would be desirable to enhance the thermal conductivity of the encapsulation material.
The organic resin used in the encapsulation of integrated circuit devices is usually selected for low moisture permeability and low thermal coefficient of expansion to avoid exposure of the encapsulated chip to moisture or mechanical stress, respectively. The choice of plastic molding materials is thus considerably narrowed. Conventionally, epoxy materials meeting these requirements have been utilized.
Cost of encapsulation materials is also a factor, particularly when encapsulation materials, such as expensive epoxy resins, are used. For this reason, as well as for enhanced thermal conductivity, fillers, such as amorphous silicon oxide (silica glass) and crystalline silicon oxide (quartz) fillers, have been used in amounts of about 68-72 percent-by-weight.
While the use of such additives reduces the cost and enhances the thermal conductivity of the encapsulant without compromising the integrity of the resin from either a strength or permeability standpoint, the thermal conductivity is still unacceptably low, particularly for VLSI densities.
Furthermore, the addition of such filler materials, which are normally naturally occurring silicas extracted from ores, e.g., quartz or the like, results in an undesirable rise in operational faults caused by alpha particle emission from trace impurities of thorium, uranium, or other radioactive materials in such ores. This problem is exacerbated by the high density of devices in VLSI technology.
Abiru et al U.S. Pat. No. 4,426,657 suggests the use of an "alpha-ray"-insulating polyimide film formed from a liquid polyimide or polyamic acid composition. The patentees suggest that the film blocks the intrusion of "alpha-rays" from naturally occurring radioactive materials found in ceramic material constituting the surrounding member, in low melting-point glass used as the sealing member, or in a solder composed of tin and lead. The film is coated on the surface of the semiconductor element to a coating thickness of from about 20 to 100 microns. The patentees state that a filler to be incorporated into the polyimide or polyamic acid solution should be of high purity s that it does not substantially produce an appreciable amount of "alpha-rays". They state that silicon is preferred as a filler because it has, in addition to the intended advantage that it imparts a suitable viscosity to the polyamic acid or polyimide solution, an advantage such that the resulting polyimide film layer exhibits an improved "alpha-ray" insulative property as compared with the polyimide film layers formed from other fillers.
While the use of such an additional coating may impart desired insulation to block intrusion of alpha radiation (alpha particles) therethrough from ceramic or glass encapsulants which contain such radiation sources as naturally occurring radioactive materials, it would be desirable to eliminate the need for such an additional coating by the provision of an encapsulant which is substantially free from such naturally occurring alpha particle radiation sources.
It would, therefore, be desirable to provide an encapsulant packaging material exhibiting little or no alpha emission, having enhanced thermal conductivity to facilitate dissipation of heat from the package, and having acceptable thermal coefficient of expansion (i.e., substantially matching that of the device), yet at an economically acceptable cost.